This invention relates to image display systems in which image elements are projected onto a display surface such as a projection screen, the display surface of a cathode ray tube or, with appropriate precautions, directly onto the retina of the human eye.
Most direct-view video projector display systems are based on cathode ray tube (CRT) technology in which a beam of electrons is directed to impinge on a phosphorescent screen. The resolution and brightness of CRT technology-based displays are inherently limited by the characteristics of phosphors and electron beam control electronics.
A projection system has been proposed using line scanning techniques, as distinct from electron pencil beam deflection techniques, as used in CRTs. An experimental video projector was described by Murano et al. entitled "A Video Projector Using a PLZT Light Shutter Array," Japanese Journal of Applied Physics, 24 (1985) Supplement 24-2, pp. 139-143. Therein a line PLZT light shutter array was used to modulate light from a xenon light source. The resulting modulated beam was scanned vertically by a movable mirror and then projected on the screen. However, this system experienced significant problems relating to brightness, uniformity of image, and scanning response time.
Laser illumination has been used in light scanning systems to illuminate a single pixel of image during a high-speed two-dimensional scan of a large projection screen system. A great deal of effort has been expended on achieving the objective using lasers because of their intense light and characteristic coherence. However, the use of lasers in wide spread commercial applications has been difficult to realize in practice for a number of reasons. Some of the difficulties have been discussed in a paper by C. E. Baker of Texas Instruments Inc., IEEE Spectrum, December 1968. One particular problem is the generally low efficiency of lasers, which results in unacceptably low picture brightness at large screen sizes for a given power consumption. An increase in laser output has involved an unacceptable increase in cost and complexity for all but the most cost-insensitive applications. Another problem involves the production of suitable red, blue and green light components for color displays. Consequently, systems employing multiple lasers have been required which results in increase in complexity and cost.
White light sources have been found to be more suitable for color projection than lasers because of the need for plurality of lasers to generate the desired colors. However, available light sources have the disadvantage of being less collimated and less coherent than a typical monochromatic laser. Thus, white light sources might be considered less suitable for spot scanning. A white light source requires relatively large components, including concentration lens, modulator and the like in order to achieve a brightness comparable to that of a laser. Large optical components are unwieldy and expensive. Moreover, large modulators are characterized by speed limitations, making them unsuitable for high-speed scanning systems. Finely-focused bright white light sources are difficult to achieve and hence, resolution is limited. Accordingly, it is desirable to provide a system which neither requires a laser nor the large and expensive optical components which have in the past generally been associated with a white light scanning system.
What is needed is a system for displaying an image from a sequence of intensity values representing pixels wherein the system is suitable for high volume, low cost production with a reliability acceptable for consumer applications of high resolution video imagery.